The present invention relates to completion of oil wells, and more particularly, to an improved method and apparatus for measuring pressure within an oil well, particularly the initial flow pressure for a well.
Initial flow pressure is a plot of absolute pressure sensed from within the borehole near the formation as a function of time at the time of formation perforation. The reaction of the hydrocarbon formation to perforation, as indicated by the initial flow pressure, may be interpreted by an engineer to provide valuable information regarding the hydrocarbon formation, including permeability of the formation, damage to the formation, whether it is necessary to treat the formation, and if so, the type of treatment required. Pressure measurements are also necessary throughout the life of a well to track reservoir depletion and evaluate continued production capability and techniques. Consequently, prompt and reliable pressure measurements are essential for proper evaluation and maintenance of an oil well.
There presently exists on the market several devices for measuring and recording downhole pressure. Most of these devices utilize downhole electronic probes communicating via electric signals to a recording apparatus at the surface. Representative of this type of device is the Lynes Sentry system, manufactured by Lynes, a Division of Baker Drilling Equipment Company, and described on pp. 10-12 of the 1982-83 Lynes Product Catalog, reprinted in IV Composite Catalogs of Oilfield Equipment and Services, 5624-26 (1982-1983).
Electronic-type downhole pressure recording systems are limited by a problem which is common to all electronic systems--electronic devices within the apparatus occasionally fail. When a failed device is located within a unit at the surface, the entire unit containing the failed device may be replaced with a backup unit and little time lost as a result. When the failed electronic device is located within a probe at the bottom of a borehole, however, the probe must be withdrawn to the surface, replaced and lowered back to the bottom of the borehole with a loss of valuable time and an increase in expense. The problem is aggravated and the expense significantly increased when the electronic-type downhole pressure recording system has previously been permanently installed on the tubing string, requiring the entire string to be withdrawn from the borehole.
Electronic-type downhole pressure recording systems are further limited by the fact that they may not be utilized at all in hot well applications, where temperatures may exceed 200 degrees centrigrade. Electronic devices within the downhole probe are typically rated for use at temperatures below 125 degrees centigrade. Use of the systems in excessive temperatures will result in failure of the electronic devices located therein.
A second type of downhole pressure recording device known as the Pressure Transmission System (PTS) is manufactured by NL Sperry-Sun, a Division of NL Industries, Inc. and described generally in the 1982-1983 Catalog of NL Sperry Sun, reprinted in IV Composite Catalogs of Oilfield Equipment and Services 6425-26 (1982-83). PTS includes a downhole chamber connected by a small-diameter tube to a surface readout unit. The chamber can either be installed permanently on the production string or suspended through a lubricator. The chamber and tube are charged with a single-phase gas (nitrogen or helium) from the surface, the downhole portion of the unit including no electronic devices. A diaphragm on the chamber compresses the enclosed gas in response to the ambient borehole pressure. A pressure gauge at the surface end of the tubing is calibrated to provide a readout of downhole pressure. Although the PTS avoids the difficulties associated with downhole electronics, the system apparently does not work so well as to persuade persons in the oil field industry to use the PTS. It appears that the sensitivity of the PTS to relatively nominal changes in pressure diminishes with increasing length of the tubing connecting the downhole chamber to the surface readout unit. This follows from the limited stroke of the diaphragm on the chamber and the increasing distance and volume of gas through which the minuscule change in pressure must be transported to the surface.
As previously discussed, a loss of time due to replacement of a failed pressure probe within the borehole increases the expense of the operation. This follows from the extreme overhead expenses associated with drilling and completion of an oil well. The expense of oil well completion thus may be limited, not only by the use of competent and reliable apparatus, as addressed above, but also by limiting the number of independent trips into the borehole which are required to complete the well. Hence, it appears that present apparatus and techniques for measuring downhole pressure do not adequately provide for certain hot well applications and can be improved upon in other cooler applications. It further appears that completion expense may be reduced if two or more independent tasks are combined to require but a single trip into the borehole.